With the continuous development of the integrated circuit technology, especially the continuous reduction of device size in scale, each of the key parameters, e.g. the threshold voltage of the device is reduced accordingly, and such advantages as reduced power consumption and increased integration promote the improvement of the overall performance of the device. But at the same time, the driving capability of the device is restricted by the conventional silicon material technology, wherein the carrier mobility is low, thus the driving capability of the device is comparatively insufficient to be used in high-speed and high-frequency application fields. Therefore, high mobility devices, especially high mobility transistors (HEMT) will be extensively used in the future.
A kind of existing high mobility field effect transistor (FET) is AlGaAs/GaAs-based, which comprises, for example, a GaAs substrate, an intrinsic GaAs layer formed on the GaAs substrate (used as a buffer layer and/or a lower cap layer), an intrinsic AlxGal-xAs layer formed on the intrinsic GaAs layer (used as a potential well layer, an active layer and a control layer), an n-doped AlxGal-xAs layer formed on the intrinsic AlxGal-xAs layer (used as an upper cap layer), a gate stack formed on the upper cap layer and source and drain (contact) regions at both sides of the gate stack. During operation of the device, electrons that function as the carriers are generally limited in the potential well layer to form two-dimensional electron gas, so the carrier mobility in the layer is greatly increased, thereby improving the driving capability of the device.
However, the above-mentioned materials and technologies are not quite compatible with the existing Si-based CMOS technology, therefore a lot of extra processes and facilities are needed when manufacturing high mobility devices, which result in a high cost. As an alternative, another kind of existing high mobility field effect transistor is made by depositing SiGe alloys of different proportions on a Si substrate as a quantum layer and using Si or SiGe as a buffer layer, a barrier layer and a cap layer. Such SiGe/Si-based high mobility FET reduces the cost, but only a limited increase of the mobility is achieved due to the limitation of the material itself.
Thus there is the need for a FET which can be manufactured simply and be of higher carrier mobility.